Ok, maybe you're trying to argue something different than what it's coming across as. Motion capture is great, green screens often aren't... these are things I agree with. And CGI is really, really good. BUT, unless you can show me an example of something that's fully CGI that looks 100% real, as if you couldn't tell the difference, then I'm not convinced. I'm not talking about something that looks incredible or something that is groundbreaking, but something that actually looks REAL. Whether you like Avatar or not, it's about as good as large scale CGI can look; the mocap in that movie is great, but still not perfect. And, for the record, the prequel trilogies DID use motion capture - both Grievous and Yoda (and probably more) were done with mocap.
As far as budgets, it's pretty clear that a lot of Benjamin Button's went towards the CGI for his face. Each of the SW prequels had a budget that was 20% smaller, and it had both a bigger cast and a hell of a lot more CGI. CGI that good is expensive, that's a fact. It's a whole lot cheaper to cast an actor than to pay hundreds of animators to do the animation.
Like I said, I have nothing against CGI. But it can't replace real people. If it's so easy or flabbergasting, then why aren't there any other movies doing it yet? Why are film studios still using real actors?